Despite recent advances made in diagnosing and treating cardiovascular disorders, coronary heart disease remains the leading cause of mortality in the United States. In 2001, 1.1 million Americans were expected to have a new or recurrent myocardial infarction. Approximately one third to one half of patients who experience their first myocardial infarction will die as a result of complications related to their acute event. Despite modern pharmacologic support, cardiogenic shock, defined as inadequate tissue perfusion resulting from a decline in cardiac output, is still a highly lethal complication of an acute myocardial infarction.
A variety of left ventricular assist devices have been developed to support the failing myocardium. The intraaortic balloon pump (IABP), which assists the ischemic ventricle through improving coronary perfusion and reducing systemic vascular resistance by counterpulsation, is by far the most widely used left ventricular assist device. The conventional IABP catheter comprises a central lumen for passage of a guidewire during insertion and for monitoring aortic blood pressure. A balloon made of polyurethane is mounted on the catheter and communicates with an outer lumen that provides a passageway for gas exchange and is connected to a console that synchronizes inflation and deflation with the cardiac cycle. The principal of counterpulsation works by deflating the balloon during systole, resulting in a reduction in systemic afterload and vascular resistance. The balloon is inflated during diastole resulting in an improvement of left ventricular performance through an increase in coronary perfusion and a decrease in myocardial oxygen consumption. Counterpulsation also causes an increase in peripheral blood flow.
Counterpulsation has been found to be successful in reversing the shock state in 80 to 85 percent of patients with cardiogenic shock after myocardial infarction. Counterpulsation is also very effective in the initial stabilization of patients with mechanical intracardiac defects complicating myocardial infarction, such as acute mitral regurgitation and ventricular septal defect. Other uses of counterpulsation include (1) treating patients with unstable angina, (2) weaning patients from cardiopulmonary bypass, (3) providing mechanical support for patients in heart failure while waiting for cardiac transplant, and (4) prophylactic application in patients with severe left ventricular dysfunction prior to or during surgery or percutaneous angioplasty.
At the present time, balloons on the IABP catheter for percutaneous insertion are available in 30 cc, 40 cc, and 50 cc volumes. Although the sizes and design of these balloons are somewhat successful in improving cardiac performance, a relatively large balloon size is required to accomplish adequate displacement of blood volume during its inflation, and to accomplish adequate backflow of blood volume during its deflation. Large balloon size is required because, during inflation, blood is displaced not only upstream (as intended for cardiac assist), but also downstream (unproductive for cardiac assist), and therefore a substantial volume of blood is dispersed to the peripheral vasculature. Likewise, during deflation, backflow occurs to draw blood not only from upstream (as intended to reduce afterload), but also from downstream (unproductive for cardiac assist), and therefore substantial backflow is wasted on drawing blood from the peripheral vasculature. Thus, new devices and methods are needed for increasing the displacement and backflow efficiency of the intraaortic balloon pump and counterpulsation, while decreasing balloon size.